Atom probe microscopes provide atomic scale structural resolution and species analysis of material surfaces. The atom probe consists of a field ion microscope used in conjunction with a time of flight mass spectrometer. Atom probes provide an atom by atom decomposition of a sharp sample “tip,” of approximately 100 nanometers in diameter. A positive voltage is applied across the sharp tip apex, to an electrode, resulting in a very strong electric field at the surface of the tip. Atoms at the surface of the tip become extended and evaporate from the surface through a physical process known as “field evaporation.” This process produces an ion near the tip which then travels toward the negative electrode. The onset of field evaporation occurs at electric fields on the order of 1–5 volts per angstrom (V/A) for all known materials (Miller, 1989). Near the field evaporation threshold, if the field is increased by a few percent, for example 20%, the field evaporation rate increases by several orders of magnitude. By pulsing the tip apex field during a short time interval, typically several nanoseconds, field evaporation is activated during a short time window. Time of flight mass spectroscopy is then performed on the evaporated atoms, thereby determining the atomic species. The width of the voltage pulse may limit the mass resolution of the atom probe (Tsong, 1982; Kellogg, 1980). The three dimensional atom probe (3DAP), or imaging atom probe, is an atom probe wherein i) the x-y position of the ion at the tip surface is mapped by projection of the field evaporated ion onto a position sensitive detector such as a microchannel plate detector or wedge and strip anode (Cerezo, 1988; Holzman, 1994), ii) the “z” position of the ion is determined by the sequence of the field evaporation event and the evaporation rate as in typical atom probes, and iii) the ion mass resolution is again provided by time of flight mass spectrometry of the ion. The application of atom probe techniques to metals and high conductivity semiconductor materials has been discussed extensively in the literature (Cerezo, 1998; King, 1994; Miller, 1989; Tsong, 1984). Further improvements in atom probe microanalysis have been facilitated by the addition of a funnel-shaped extraction electrode placed near the tip (Nishikawa, et al, 1995; Kelly, 1995). This allows atom probe analysis of individual tips in a field of tips and scanning of the atom probe over larger areas.
The main problem with conventional electrically pulsed atom probes is that they cannot be used to investigate samples with resistivity greater than approximately 10 Ohm-centimeter. As noted, the field evaporation mechanism required for atom probe operation will occur at electric fields on the order of several volts per angstrom for all materials—metals, semiconductors, or dielectrics. However, dielectrics and common semiconductors do not possess the conductivity required to support the short voltage pulsing needed to attain acceptable time of flight resolution, thus the atomic species cannot be resolved for dielectrics and common semiconductors using conventional voltage pulsed atom probes. For this reason conventional voltage pulsed atom probes are limited to “high-conductivity” applications such as metals or low resistivity semiconductor materials. Electrically pulsed atom probes also typically encounter difficulty in operation on samples of resistivity approximately 1 Ohm-centimeter or greater. For n-type silicon, 1 Ohm-centimeter resistivity corresponds to a dopant concentration of approximately 5×1015 atoms per cubic centimeter, while 10 Ohm-centimeter resistivity corresponds to a dopant concentration of approximately 5×1014 atoms per cubic centimeter. Thus, atom probe imaging of silicon samples with dopant levels below 5×1015 atoms per cubic centimeter (cm3) becomes difficult, and imaging of silicon samples with dopant levels below 5×1014 atoms per cm3 is impossible. This “conductivity limit” is the principle impediment to the application of atom probe techniques to the dielectric and semiconductor materials commonly used in microelectronic and optoelectronic devices.
The inability to transmit short, high voltage pulses through semiconductor and dielectric tips has been overcome in a number of experiments by using laser pulsing to deposit thermal energy into the tip, thereby activating the field evaporation mechanism through an increase in temperature (King et al., 1994; Tsong et al., 1982). However, a substantial problem with conventional pulsed laser atom probes is that they fail to use an ultraviolet (UV) or visible wavelength laser wherein efficient thermal pumping of field evaporation in semiconductor materials is attained. Generally, optical absorption in semiconductor materials is peaked toward the ultraviolet. Thus, the efficiency of thermal pulsing of semiconductor and dielectric materials is improved by using UV laser wavelengths where the optical absorption is strong. However, prior studies of thermally pumped field evaporation in semiconductors have utilized laser wavelengths in the near-infrared, where the optical absorption is weak (King, 1995). Another problem with conventional electrically pulsed atom probes is that the use of high voltage pulsing on low conductivity samples induces ion energy dispersion (Cerezo, 1998) and tip mechanical stress failure. The pulsed laser approach also minimizes energy dispersion and mechanical stress. Thus, the pulsed laser atom probe also has advantages with respect to mass resolution and reliability. Also, although there have been a number of pulsed laser atom probe studies on metals (Kellogg, 1980), no significant advantage is conferred by using a pulsed laser on metal samples since metals may be analyzed using the conventional voltage pulsed atom probe technique.
An additional problem with conventional pulsed laser atom probes is they fail to utilize a laser wavelength located near an optical absorption edge. In this case the optical absorption coefficient will depend strongly on the static field, as the absorption coefficient undergoes a substantial redshift in strong electric fields (Keldysh, 1958). Thus, the optical absorption coefficient may be enhanced through an electric field induced redshift of the optical absorption, provided the laser wavelength is chosen nearby to strong absorption features. This dependence of the optical absorption on electric field has never been utilized in the prior art pulsed laser atom probe technique.
However, despite the aforementioned deficiencies in the prior art, the most significant problem with the conventional pulsed laser atom probe technique is that it does not utilize photo-ionization for atom probe imaging of semiconductor and dielectric materials. The ability to directly photo-ionize atoms from the surface of an emission tip provides a major advance in the atom probe microanalysis technique. For example, photo-ionization allows the accurate imaging of molecular complexes such as viruses, proteins and organic molecules embedded in a sample tip. This is facilitated by the reduced distortion of surface atoms when using photo-ionization for atom probe imaging of such complexes. Although prior studies have failed to realize this photo-ionization mechanism, it is anticipated in semiconductors and dielectrics due to the much slower neutralization of surface ions than found in metals (Kellogg, 1980; Tsong, 1976).
Thus, while these devices may be suitable for the particular purpose to which they address, they are not as suitable for atom probe imaging of dielectric and low conductivity semiconductor materials. In these respects, the laser stimulated atom probe technique according to the present disclosure substantially departs from the conventional concepts and designs of the prior art, and in so doing provides a technique primarily developed for the purpose of atom probe imaging of semiconductor and dielectric materials.